docs: Add user guide for alert words.

templates/zerver/help/alert-words.md

@@ -0,0 +1,33 @@
+# Alert Words
+Zulip allows you to define custom alert words and phrases.
+When they are used in any stream you are subscribed to, it works
+similarly to being [@-mentioned](/help/at-mention-a-team-member)
+by another user.
+
+## Adding Alert Words
+1.  Click the cog (![cog](/static/images/help/cog.png)) in the upper
+right corner of the right sidebar.
+2.  Select **Settings** from the dropdown menu that appears.
+    ![Settings dropdown](/static/images/help/settings-dropdown-menu.png)
+
+3.  Scroll down until you reach the **Custom Alert Words** section.
+
+    ![Custom Alert Words](/static/images/help/settings-custom-alerts.png)
+
+4.  Type the word/phrase you wish to be notified for into the text box,
+    then click the **Add Alert Word** button.
+
+Once these steps are completed, you should see something similar to:
+
+![Alert Settings](/static/images/help/settings-custom-alerts-sample.png)
+
+If any alert words are mentioned, they will be highlighted within the message,
+and you will be alerted in accordance with your notification settings.
+
+![Example Notification](/static/images/help/alert-word-notification-example.png)
+
+## Removing Alert Words
+
+Alert Words can be removed easily in the settings menu, by clicking the
+red bin button (![Trash Can](/static/images/help/delete-button.png))
+next to the word/phrase to be deleted.

templates/zerver/help/index.md

@@ -110,7 +110,7 @@ as a “**realm**”.
 * Configure audible notifications
 * Configure email notifications
 * Configure mobile push notifications
-* Add an alert word
+* [Add an alert word](/help/alert-words)
 
 ## Tools & Customization
 * [Keyboard shortcuts](/help/keyboard-shortcuts)